The present invention relates, in general, to packet data communication systems and, more specifically, to an apparatus, method and system for a dual polling media access control protocol for packet data in fixed wireless local loop CDMA-based communication systems.
Fixed wireless communication systems are becoming increasingly viable solutions for providing local loop telecommunication network access, particularly in developing nations. The alternative, wireline local loop access, may be prohibitively expensive, with a sparse availability of wireline telecommunication systems operating as a constraint on economic development. As a consequence, telecommunication service providers are increasingly turning to wireless technologies, with or without wireline services, to implement a cost-effective local loop telecommunication service, for voice, data, and other multimedia uses.
A wireless local loop (xe2x80x9cWLLxe2x80x9d) system typically includes a base station which transmits to and receives from various transceivers (within subscriber units) which are located at each home, business or other customer premises being served. The base station broadcasts to all the subscriber units it serves (xe2x80x9cforwardxe2x80x9d, xe2x80x9cdownlinkxe2x80x9d or xe2x80x9cdownstreamxe2x80x9d direction). The broadcast includes an identification of the intended subscriber unit, which then selects that broadcasted information for the particular customer premises. Similarly, when it has information to send, each subscriber unit transmits the information to the base station (xe2x80x9creversexe2x80x9d, xe2x80x9cuplinkxe2x80x9d or xe2x80x9cupstreamxe2x80x9d direction). These upstream transmissions are typically separated by time (time division multiple access (TDMA)), frequency (frequency division multiple access (FDMA)), or code (code division multiple access (CDMA)), creating multiple channels which, to some degree, prevent the subscriber units from contending for the same resources. As all subscriber units are not anticipated to be constantly transmitting, for cost-effectiveness and other efficiency considerations, system providers may allocate system resources so that upstream channels are shared among multiple subscriber units, creating an inherent potential for conflict on each such shared upstream channel.
As a consequence, for packet data systems with a number of subscribers sharing the same radio or channel resources, a protocol is needed to resolve the potential contention among subscriber units for the available upstream channels. Existing CDMA-based packet radio systems utilize a random access media access control (xe2x80x9cMACxe2x80x9d) protocol, in which a transmission of a single data packet in a given time period is generally received correctly, while simultaneous transmission of an additional packet causes a collision, with all packets destroyed.
One such random access protocol is ALOHA (see, e.g., A. Tanenbaum, Computer Networks (3d. ed. 1996) at 246-50). When data arrives at the subscriber unit for upstream transmission, such as from a computer within the customer premises, it is transmitted immediately. When the base station receives an uncorrupted packet, the base station broadcasts an acknowledgement to the sender. If no acknowledgement is received, the transceiver xe2x80x9cbacks offxe2x80x9d for a random period of time, and then retransmits the packet. If two or more transmissions overlap in time (collide), all are corrupted, and the base station does not acknowledge any of them. Another variant of ALOHA, slotted ALOHA, provides for upstream transmissions in unassigned time slots, but still has a significant probability of collisions within these time slots.
Most random access protocols, such as ALOHA and its variants, are inherently unstable, and statistical fluctuations may easily cause their saturation. These protocols tend to reach a situation in which the percentage of sources attempting to retransmit approaches one hundred percent, while the throughput approaches zero. In addition, with this inherent instability, a system with a random access protocol must be periodically reset to operate in its stable region of its throughput-channel traffic rate curve. As a consequence, while delay and throughput characteristics may be satisfactory in the short term, they are quite poor over a long period of time.
Additionally, protocols such as ALOHA require a very low loading to work with any efficiency. They require a small packet size, or the probability of collision becomes excessively high, and have a low maximum throughput (e.g., approximately 18%). In addition, there is no theoretical bound for transmission delay, especially for users with comparatively infrequent or small data transmissions.
Other protocols, which may be suitable for wireline or cable applications, are unsuitable for fixed wireless applications. For example, in Carrier Sense Multiple, Access (CSMA) and Digital Sense Multiple Access (DSMA), when a user has data to transmit, the transceiver looks for current activity on the selected channel, and if there is no activity, it transmits. For radio applications, this would rely upon time division duplexing, as corresponding transceivers (within subscriber units) would have to both transmit and receive on the same frequency. For fixed wireless applications, there is an additional difficulty because the transceivers are directional and cannot directly receive transmissions from other transceivers, making direct carrier sense impossible.
In DSMA, activity on the uplink (upstream) channel is broadcast by the a base station to the subscriber units on the downlink channel(s). As a consequence, the effectiveness of DSMA in preventing data collisions depends upon the accuracy of the sensing operation by the transceivers. In addition, there is a period of vulnerability, corresponding to the time required for the broadcast to be received by all subscriber units. During this period, other users may still perceive the channel as idle, start a transmission, and cause a collision.
Another difficulty for any desired protocol arises within CDMA-based communication systems, namely, power control. More specifically, any packet data protocol for use with CDMA requires power control over the various transceivers, to have the same received power at the base station, to avoid increased noise levels and interference with other transceivers. In circuit switched systems, such as CDMA-based voice telephony, upstream transmission is effectively continuous, for the entire duration of the communication session, allowing correspondingly continuous power measurement and power correction. In contrast, packet data transmission is generally bursty, with transceivers transmitting for a short duration, followed by no transmission, followed by another transmission burst, and so on. A mechanism is needed in a wireless protocol for dynamic power control, to account for highly variable transmission characteristics and to accommodate bursty traffic patterns.
As a consequence, a need remains for a deterministic protocol for upstream or uplink packet data transmission in a wireless local loop communication system. Such a protocol should have guaranteed maximum delay and minimum throughput characteristics. Such a protocol should provide for maximal throughput, with minimum delay, for delay-sensitive data packets. The various embodiments of the deterministic protocol should also provide power control, for fixed wireless transmission systems, during data transmission.
In accordance with the present invention, an apparatus, method and system are provided for a dual polling media access control (xe2x80x9cMACxe2x80x9d) protocol, which has guaranteed maximum delay and minimum throughput characteristics. The various embodiments of the deterministic protocol of the present invention also provide for power control, for fixed wireless transmission systems, both during data transmission and in between data transmissions. As a consequence, when data is transmitted, the initial power setting is approximately correct.
The preferred system embodiment includes a plurality of subscriber units, a base station having a transceiver for wireless communication with the plurality of subscriber units, and a data interface unit operatively coupled, through a network access controller, to the base station. The network access controller splits or divides the voice and data services provided to the subscriber. The voice services are directed to a public switched telephone network (xe2x80x9cPSTNxe2x80x9d), and data services are carried through a data interface unit (DIU) to a packet network, such as the Internet. Multimedia and other data packets are transmitted in a downstream or downlink direction, from a network, through the data interface unit, network access controller and base station to a subscriber unit, while data packets are transmitted in an upstream or uplink direction, from a subscriber unit, through the base station, network access controller and data interface unit, to the network.
The upstream and downstream directions operate -at different carrier frequencies using frequency division duplex (xe2x80x9cFDDxe2x80x9d). In each direction, a packet data channel is shared among a number of subscriber units using a MAC protocol, with the protocol of the present invention applicable to upstream transmissions. Each packet data channel is divided into two (or more) subchannels for both upstream and downstream communications. As discussed in greater detail below, to control potential conflict among subscriber units transmitting data in the upstream direction, the protocol of the present invention utilizes two types of polls (hence, xe2x80x9cdualxe2x80x9d polling), to resolve or avoid any potential contention between subscriber units in the upstream direction for the two or more subchannels. The first type of poll, referred to as a standard or first poll, is transmitted by the DIU (via the network access controller and base station) sequentially (round-robin) to each subscriber unit assigned to a given channel. In response to a standard poll, a subscriber unit may transmit data, up to a first predetermined number of bytes. If the subscriber unit has no data to transmit at that time, it transmits a xe2x80x9cno data messagexe2x80x9d in the poll response. The first predetermined number of bytes allowed to be transmitted by a standard poll, the xe2x80x9cstandard allocationxe2x80x9d, is comparatively small, and is designed for prompt throughput of relatively small but delay sensitive messages, such as acknowledgement messages. Following reception of a poll response with either a no data message or a data packet (not to exceed the standard allocation of data), or following a time out if no poll response is received, another standard poll is transmitted to the next subscriber unit assigned to the channel, and so on, in round-robin fashion. By limiting the amount of data that can be transmitted in response to a standard poll, and thereby limiting the amount of time that a given channel is used by a given subscriber unit, the present invention provides the means to rapidly poll all the subscribers and allow the transmission of many short messages from multiple subscribers. In addition, by having each subscriber unit transmit on a regular basis, transmitting either data in a poll response or a no data message, power control is facilitated, as discussed in greater detail below.
If the subscriber unit has additional information to transmit, i.e., more information than the first predetermined number of bytes allowed in the standard allocation, then along with its standard allocation of data, the subscriber unit includes such an indication in. its poll response, such as through a flag or parameter indicating additional (stored) data remaining for transmission. In addition to standard polls, a second type of poll, referred to as an extended poll, is then transmitted sequentially (round-robin) to the subset of subscriber units having such additional data to transmit, preferably on-one selected subchannel (of the two or more available subchannels). In response to an extended poll, a subscriber unit may transmit data, up to a second predetermined number of bytes. The second predetermined number of bytes allowed to be transmitted by an extended poll, the xe2x80x9cextended allocationxe2x80x9d, is comparatively large, and is designed for significant throughput of comparatively large but not delay sensitive messages, such as email messages, attachments and other file uploads. In the preferred embodiment, the extended allocation is 1550 bytes, compared to the standard allocation of 256 bytes. In addition, while receiving extended polls allowing upstream transmission of an extended allocation, the subset of subscriber units is also receiving standard polls, allowing additional upstream transmission of smaller size data packets as well.
As a subscriber unit receives these various standard polls and extended polls, it divides or fragments the data it has for transmission, in order to transmit the appropriate amount authorized by the given type of poll, until it has no further data for transmission. Conversely, the subscriber unit may also aggregate several smaller data packets to transmit the maximum amount authorized by a given poll. The fragmentation or aggregation of data for transmission is variable, depending upon any given received sequence of standard or extended polls, with corresponding amounts of data fragmented or aggregated for transmission in a poll response. For example, depending upon the status of the various extended and standard polling lists at any particular time, a given subscriber unit may receive a standard poll, followed by an extended poll, followed by one standard poll and one extended poll, with corresponding fragmentations or aggregations of one small block of data, one larger block of data, one small block of data and one larger block of data.
Improved power control is also a significant feature of the present invention. Following synchronization and signal acquisition bits, in the preferred embodiment, each response to standard or extended polls includes a preamble for power measurement to perform power control. During reception of the response, power measurement is performed at the base station. Any needed changes in power levels are determined immediately, with a power control message transmitted by the base station to the subscriber unit on a downlink channel, in time for the power control to take effect prior to transmission of a data payload in the same response message. (The no data message itself is utilized for power measurement and, as a consequence, does not include a separate power measurement preamble.) In addition, based upon power control instructions from previous responses to standard polls, any given subscriber unit has a recent estimate of appropriate power levels, for use in response to either another standard poll or an extended poll, increasing the probability of a successful, uncorrupted transmission. Such power control is particularly useful for the preferred embodiment utilizing CDMA for wireless transmission.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.